Tank experiments

In 1973 when the Snaggletooth series of Mustangs was designed, I considered every feature of stunt ships including tank design and installation. typically, tanks of the period ran well enough but but caused problems when the tanks were nearly empty. The usual engine run would terminate with several laps of unsteady engine operation with the engine surging or quiting when flying on the upwind side of the circle. I reasoned this effect was due to there being some fuel remaining in the tank which would run to the front when the wind cause the tail to drift inward (yaw) on the upwind side of the circle temporarily uncovering the pickup at the back. It was also reasonable to assume that with the typical tank installation there would almost always be fuel remaining in the front of the tank because our airplanes fly with a bit of outward yaw all the time. If the tank is installed so that is is aligned with the flight path instead of fuselage centerline, there would be less residual fuel at the end of the flight. What little remained could be forced to the front of the tank with gyroscopicly induced outward yaw from a tight inside loop to cause the engine to quit almost on demand. This worked exceptionally well when the rear of the tank was canted 1/4" outward at the back as installed in all of the Snaggletooth series airplanes. I could reliably force the engine to quit within three laps of finishing the pattern. this capability was so apparent that many airplanes today use canted tanks. This discussion was published in the 1978 article "Evolution...".

This didn't end the tank experiments with Snaggletooth. The airplane was originally designed with a 1 1/4" deep, 2" wide tank containing about 6 ounces of fuel. I cut twice into the airplane to install larger tanks as larger or more powerful engines were installed in the airplane. The last major surgery was performed two days before leaving for the 1978 World Champs in Liverpool. The reason for the surgery was that I just crashed my special built World Champ Mustang VI and Snaggletooth didn't have enough tank to take advantage of the engine power I was getting at the time. It worked. I had no complaints about my engine runs in Liverpool.

When I got home I started building my own engines from parts from other engines and my own bar stock crankshafts, connecting rods, pinned rings, heads and combustion chamber buttons. I managed to stick a true 60 into a Como 40 case to fit ST 46 mounting. The case had to be machined to .005 wall thickness to clear the rod and was basically held together by the engine lugs. With another increase in power, I was again short of fuel. To solve this, I built an axillary tank in the engine compartment between the engine and the firewall. It was a temporary construction of balsa and epoxy, intended to last only long enough to prove the theory that an aux tank could work.

Operation of the tank system worked like this. Muffler pressure into the aux tank would force the aux tank fuel through a pickup line connected to the original muffler pressure vent so that as fuel was drawn from the main tank it was replaced with aux tank fuel until the aux fuel was exhausted, at which time muffler pressure would flow through the aux tank pickup to the main tank as though the aux tank didn't exist. the tank could be fueled normally with the aux tank being filled from the main tank. No modifications to the original installation were necessary.

It worked perfectly. I picked up about a minute and half from the extra fuel with no discernible change in the way the engine ran throughout the pattern. I considered the experiment a complete success and intended to install a metal tank permanently. I cleaned the engine compartment of the temporary tank but never got around to building the aux tank, so Snagletooth hangs on the wall unflyable, with bare wood exposed in the tank compartment. this convinced me that fuel need not be all contained in a traditional tank. The aux tank was installed in 1979 but reported only in forums like this. It didn't seem, by itself, sufficient for another magizing article.

SO, You have a main tank which your plane holds fuel. Then you have another tank that the engine DRAWS from……….Head pressure would remain constant through out the engine run UNTILL main tank has ZERO head….RIGHT? kool.

Scott(ideas run ) Riese

Not quite. All the fuel was always drawn from the main tank, with or without the aux tank. This required no change to the interior of the main tank, just a temporary addition the the muffler pressure vent ahead of the firewall inside the aux tank. Basically, muffler pressure pushed fuel out of the aux tank into the main tank until the aux tank was empty, then reverted to normal muffler pressure operation. Apparently the engine didn't see any difference in head pressure when the fuel was being transfered and when the main tank reverted to normal muffler pressurized operation. When you stop to think about it, the system was always muffler pressurized, even though it began in the aux tank. The movement of fuel from the aux tank to the main tank didn't offer enough resistance to change the fuel draw from the main tank. To the engine, it simply looked like a somewhat larger tank utilizing muffler pressure.

>Not quite. All the fuel was always drawn from the main
>tank, with or without the aux tank. This required no change
>to the interior of the main tank, just a temporary addition
>the the muffler pressure vent ahead of the firewall inside
>the aux tank. Basically, muffler pressure pushed fuel out
>of the aux tank into the main tank until the aux tank was
>empty, then reverted to normal muffler pressure operation.
>Apparently the engine didn't see any difference in head
>pressure when the fuel was being transfered and when the
>main tank reverted to normal muffler pressurized operation.
>When you stop to think about it, the system was always
>muffler pressurized, even though it began in the aux tank.
>The movement of fuel from the aux tank to the main tank
>didn't offer enough resistance to change the fuel draw from
>the main tank. To the engine, it simply looked like a
>somewhat larger tank utilizing muffler pressure.
>
>Al

Al,

Can the extender tank sit on top of the regular tank ? Clearly it's height wouldn't matter once the extender tank was empty. I am thinking about using the stock tank in my new Nobler ARF and then adding a one ounce tank above it. Might work better than the 6 ounce clunk tank I have been considering.

I replaced a FOX .35 with a Double Star .40. The venturi in the Double Star .40 was bored out to .312" and requred more than 4 oz of fuel. I discovered that the tank was built in the Dolphin and could not take a larger tank than 4 oz without some major surgery.
To solve the problem, I cut off about 1" of the back of another tank and ended up with a mini-tank of 1" tall, 2" wide and 1" in length with the one end open. Then cut into the Dolphin and removed the old 4 oz tank. Used an ice pick to put holes in the old tank on the inboard side and soldered the mini-tank onto the bottom of the old tank covering up the holes in the old tank.

I always try to prefuel the tank with about 2 1/2 oz of fuel, to give the fuel time to work its way done to the bottom mini-tank. Then fill up the rest of the tank just before starting. I have not notice any change in the run characteristics of the engine. With the extra fuel I get a run time of about 6:30.

>SO, You have a main tank which your plane holds fuel. Then
>you have another tank that the engine DRAWS from……….Head
>pressure would remain constant through out the engine run
>UNTILL main tank has ZERO head….RIGHT? kool.
>
>Scott(ideas run ) Riese

I believe the tank that you are referring to Scott is a header tank. If I am correct the RC Pattern guys use or used to use this type of setup to ensure a steady flow of fuel to the engine. Works just like you stated. The engine would draw from the header tank and would always be full until such time as the main tank was empty. I always wondered if anyone had ever tried this type of setup but thought that it may add unneccessary additional weight.

Finch,

Yes, Scott was referring to a header tank. This wasn't a header tank. It was an aux tank. All fuel was drawn from the main tank. There have been many unusual tanks in R/C and combat to say nothing of "chicken hoppers" and bladders, but this was truly an aux tank in a stunt ship.

>Finch,
>
>Yes, Scott was referring to a header tank. This wasn't a
>header tank. It was an aux tank. All fuel was drawn from
>the main tank. There have been many unusual tanks in R/C
>and combat to say nothing of "chicken hoppers" and bladders,
>but this was truly an aux tank in a stunt ship.
>
>Al

AL, Finch

Al, now I get it. I didn't see the Muffler pressure part of this. Finch, YES I was thinking on a header tank. With Uniflow we as stunt flyers don't need a header. But SPEED? I know nothing about speed repeat NOTHING. Fluid pressures inside a speed tank would be fun to exhume.

Scott(like the slow planes)Riese

Great Post......."THANKS AL".....Married with Children

>compartment between the engine and the firewall. It was a
>temporary construction of balsa and epoxy, intended to last
>only long enough to prove the theory that an aux tank could
>work.
Out of curiocity, how come balsa tanks are not used? They would be lighter than metal ones. They can be made integral part of the engine crutch thus increasing nose rigidity. One could glass the interior to prevent fuel from escaping.

I think it would be impossible to completely fuel proof an installed boxlike tank, inside and outside. I don't think it is possible to build a balsa and epoxy tank without using exotic epoxies, if then. Commonly available resins are not sufficiently fuel proof. I certainly tried, but failed. The tank was soaked after one flying session in spite of my best efforts to make the tank at least semi-permanent. Still, I learned about the possibilities of aux tanks and was satisfied with the experiment.

My experience with the limited capability of "normal" epoxies to resist fuel immersion agrees with Al's. Kind of makes you wonder what Windy uses on his Carbon Fiber tanks...

>My experience with the limited capability of "normal"
>epoxies to resist fuel immersion agrees with Al's. Kind of
>makes you wonder what Windy uses on his Carbon Fiber
>tanks...

Possibly vinyl esters or some other exotic resins. The alcohol in glow fuel will destroy epoxies in a very short time, although you can use reinforced epoxy tanks for Diesel fuel or petrol based fuels with no trouble at all.

Charlie

http://home.wxs.nl/~wakke007/

Real good article on balsa diesel tanks.

>Possibly vinyl esters or some other exotic resins. The
>alcohol in glow fuel will destroy epoxies in a very short
>time

Then how come it is generally recomended to coat the engine compartment with thinned epoxy if it will be disolved anyways?!

Steve.. The alcohol rapidly evaporate hence no real attack on the epoxy. Very different from having a tank holding alcohol.